1. Field of the Invention
The present invention relates to an optical pickup apparatus configured to perform an operation of reading signals recorded in an optical disc and an operation of recording signals into an optical disc using a laser beam.
2. Description of the Related Art
Optical disc apparatuses are widely used that are capable of performing a signal reading operation and a signal recording operation by irradiating a signal recording layer of an optical disc with a laser beam emitted from an optical pickup apparatus.
Although optical disc apparatuses that use optical discs called CD and DVD are generally in widespread use, optical disc apparatuses are recently developed that use optical discs with record densities improved, i.e., optical discs of the Blu-ray standard.
Infrared light having a wavelength of 785 nm is used as a laser beam for performing the operation of reading signals recorded in a CD-standard optical disc, and red light having a wavelength of 665 nm is used as a laser beam for performing the operation of reading signals recorded in a DVD-standard optical disc.
A transparent protective layer provided between a signal recording layer and an optical disc surface of a CD-standard optical disc has a thickness of 1.2 mm, and a numerical aperture is set to 0.47 in an objective lens used for performing the operation of reading signals from this signal recoding layer. A transparent protective layer provided between a signal recording layer and an optical disc surface of a DVD-standard optical disc has a thickness of 0.6 mm, and a numerical aperture is set to 0.6 in an objective lens used for performing the operation of reading signals from this signal recoding layer.
In contrast to the CD-standard and DVD-standard optical discs, a laser beam having a shorter wave length, such as blue-violet light having a wavelength of 405 nm, is used as a laser beam for performing the operation of reading signals recorded in a Blu-ray-standard optical disc.
A protective layer provided on a top surface of a signal recording layer of the Blu-ray-standard optical disc has a thickness of 0.1 mm, and a numerical aperture is set to 0.85 in an objective lens used for performing the operation of reading signals from the signal recording layer.
It is necessary to reduce the diameter of a laser spot formed by condensing a laser beam so as to read signals recorded on the signal recording layer and recording signals onto the signal recording layer provided in a Blu-ray-standard optical disc. An objective lens used for acquiring a desired laser spot shape has not only an increased numerical aperture but also a shortened focal length, and is therefore characterized by a reduced curvature radius of the objective lens.
An optical disc apparatus has been commercialized that is capable of performing the operation of reading signals recorded in all the optical discs of the CD standard, the DVD standard, and the Blu-ray standard described above, and the operation of recording signals thereinto, and as an optical pickup apparatus built into such an optical disc apparatus, an optical pickup apparatus is generally employed that incorporates: a laser diode configured to emit a first laser beam for performing the operation of reading signals recorded in a Blu-ray-standard optical disc; a first objective lens configured to condense the first laser beam emitted from the laser diode onto a signal recording layer; a two-wavelength laser diode configured to emit a second laser beam for performing the operation of reading signals recorded in a DVD-standard optical disc and a third laser beam for performing the operation of reading signals recorded in a CD-standard optical disc; and a second objective lens configured to condense the second laser beam and the third laser beam onto signal recording layers of the respective optical discs (see Japanese Laid-Open Patent Publication No. 2010-61781).
In order to miniaturize an optical pickup apparatus configured to perform the operation of reading signals recorded in three types of optical discs compatible with different standards using a laser diode configured to emit a laser beam having one wavelength, a two-wavelength laser diode emitting laser beams having two wavelengths, and two objective lenses, commonality of optical paths of the first laser beam, the second laser beam, and the third laser beam is provided. An optical pickup apparatus with such a configuration will be described with reference to FIGS. 4 and 5.
In FIG. 4, reference numeral 1 denotes a laser diode configured to generate and emit a first laser beam of blue-violet light having a wavelength of 405 nm, for example, and reference numeral 2 denotes a first diffraction grating on which the first laser beam emitted from the laser diode 1 is incident, and which includes: a diffraction grating unit 2a configured to divide the laser beam into a main beam of zero-order light and two sub-beams of plus first-order light and minus first-order light; and a half-wave plate 2b configured to convert an incident laser beam into S-linearly-polarized light.
Reference numeral 3 denotes a two-wavelength laser diode in which a first laser element, configured to generate and emit a second laser beam of red light having a wavelength of 655 nm, and a second laser element, configured to generate and emit a third laser beam of infrared light having a wavelength of 785 nm, for example, are housed in the same case.
Reference numeral 4 denotes a second diffraction grating on which the second laser beam emitted from the first laser element and the third laser beam emitted from the second laser element, the elements built into the two-wavelength laser diode 3, are incident, and which includes: a diffraction grating unit 4a configured to divide the laser beam into a main beam of zero-order light and two sub-beams of plus first-order light and minus first-order light; and a half-wave plate 4b configured to convert an incident laser beam into S-linearly-polarized light.
Reference numeral 5 denotes a divergence lens which is provided in a position where the second laser beam and the third laser beam emitted from the two-wavelength laser diode 3 are incident through the second diffraction grating 4, and which has a function of adjusting a divergent angle of the incident laser beams of diverging light.
Reference numeral 6 denotes a semitransparent mirror configured to reflect the S-polarized light of the first laser light having passed through the first diffraction grating 2 and incident thereon as well as allow the P-polarized light, which is return light of the first laser beam, the second laser beam, and the third laser beam reflected from an optical disc through an optical path described later, to pass therethrough. Reference numeral 7 denotes a polarizing beam splitter configured to reflect the S-polarized light of the second laser beam and the third laser beam incident thereon through the second diffraction grating 4 and the divergence lens 5, allow the first laser beam, having been reflected by the semitransparent mirror 6 and incident thereon, to pass therethrough, and allow the P-polarized light, which is return light of the first laser beam, the second laser beam, and the third laser beam reflected from an optical disc, to pass therethrough.
Reference numeral 8 denotes a three-wavelength compatible quarter-wave plate, which is provided in a position where the first laser beam, having passed through the polarizing beam splitter 7, and the second and the third laser beams, reflected by the polarizing beam splitter 7, are incident, and which has an effect of converting the incident laser beam from linearly-polarized light into circularly-polarized light and reversely from circularly-polarized light into linearly-polarized light according to laser beams having three different wavelengths.
Reference numeral 9 denotes a collimating lens on which the laser beam having passed through the quarter-wave plate 8 is incident and which is configured to convert the incident laser beam into parallel light, and is also configured to correct the spherical aberration caused by an operation of displacing the collimating lens 9 in an optical axis direction due to a thickness of a protective layer of an optical disc.
Reference numeral 10 denotes a first objective lens configured to condense the first laser light onto a signal recording layer L1 provided in a first optical disc D1 (see FIG. 5), and reference numeral 11 denotes a second objective lens compatible with two wavelengths configured to condense the second laser beam onto a signal recording layer L2 provided in a second optical disc D2 and condense the third laser beam onto a signal recording layer L3 provided in a third optical disc D3. In such a configuration, the first objective lens 10 and the second objective lens 11 are mounted on a member called a lens holder supported by four support wires so as to enable a displacement operation in a focusing direction, which is a direction orthogonal to a surface of an optical disc, and a displacement operation in a tracking direction, which is a radial direction of an optical disc, for example.
The first laser beam, the second laser beam, and the third laser beam having passed through the collimating lens 9 are guided by an optical system depicted in FIG. 5 to the first objective lens 10 and the second objective lens 11. In FIG. 5, reference numeral 12 denotes a wavelength selectivity element configured to allow the first laser beam to pass therethrough and reflect the second laser beam and the third laser beam in the direction of the second objective lens 11. Reference numeral 13 denotes a raising mirror configured to reflect the first laser beam having passed through the wavelength selective element 12 in the direction of the first objective lens 10.
In such a configuration, the first laser beam having passed through the collimating lens 9 passes through the wavelength selective element 12 and is reflected by the raising mirror 13, to be made incident on the first objective lens 10. The first laser beam incident on the first objective lens 10 as such is condensed by the focusing operation of the first objective lens 10 onto the signal recording layer L1 provided in the first optical disc D1.
The second laser beam having passed through the collimating lens 9 is reflected by the wavelength selective element 12 and is incident on the second objective lens 11. The second laser beam incident on the second objective lens 11 as such is condensed by the focusing operation of the second objective lens 11 onto the signal recording layer L2 provided in the second optical disc D2. The third laser beam having passed through the collimating lens 9 is reflected by the wavelength selective element 12 and incident on the second objective lens 11. The third laser beam incident on the second objective lens 11 as such is condensed by the focusing operation of the second objective lens 11 onto the signal recording layer L3 provided in the third optical disc D3.
In such a configuration, the first laser beam emitted from the laser diode 1 is incident, via the first diffraction grating 2, the semitransparent mirror 6, the polarizing beam splitter 7, the quarter-wave plate 8, the collimating lens 9, the wavelength selective element 12, and the raising mirror 13, on the first objective lens 10, and thereafter is applied as an irradiation spot by the focusing operation of the first objective lens 10 onto the signal recording layer L1 provided in the first optical disc D1, and the first laser beam applied to the signal recording layer L1 is reflected as return light by the signal recording layer L1.
The second laser beam emitted from the first laser element of the two-wavelength laser diode 3 is incident, via the second diffraction grating 4, the divergence lens 5, the polarizing beam splitter 7, the quarter-wave plate 8, the collimating lens 9, and the wavelength selective element 12, on the second objective lens 11, and thereafter is applied as an irradiation spot by the focusing operation of the second objective lens 11 onto the signal recording layer L2 provided in the second optical disc D2, and the second laser beam applied to the signal recording layer L2 is reflected as return light by the signal recording layer L2.
The third laser beam emitted from the second laser element of the two-wavelength laser diode 3 is incident, via the second diffraction grating 4, the divergence lens 5, the polarizing beam splitter 7, the quarter-wave plate 8, the collimating lens 9, and the wavelength selective element 12, on the second objective lens 11, and thereafter is applied as an irradiation spot by the focusing operation of the second objective lens 11 onto the signal recording layer L3 provided in the third optical disc D3, and the third laser beam applied to the signal recording layer L3 is reflected as return light by the signal recording layer L3.
The return light of the first laser beam reflected from the signal recording layer L1 of the first optical disc D1 is incident, via the first objective lens 10, the raising mirror 13, the wavelength selective element 12, the collimating lens 9, the quarter-wave plate 8, and the polarizing beam splitter 7, on the semitransparent mirror 6. The return light incident on the semitransparent mirror 6 as such has been changed into linearly-polarized light in the P-direction by the phase shift operation of the quarter-wave plate 8. Therefore, the return light of the first laser beam is not reflected by the semitransparent mirror 6 and passes through the semitransparent mirror 6 as a control laser beam.
The return light of the second laser beam reflected from the signal recording layer L2 of the second optical disc D2 is incident on the semitransparent mirror 6 through the second objective lens 11, the wavelength selective element 12, the collimating lens 9, the quarter-wave plate 8, and the polarizing beam splitter 7. The return light incident on the semitransparent mirror 6 as such has been changed into linearly-polarized light in the P-direction by the phase shift operation of the quarter-wave plate 8. Therefore, the return light of the second laser beam is not reflected by the semitransparent mirror 6 and passes through the semitransparent mirror 6 as a control laser beam.
The return light of the third laser beam reflected from the signal recording layer L3 of the third optical disc D3 is incident, via the second objective lens 11, the wavelength selective element 12, the collimating lens 9, the quarter-wave plate 8, and the polarizing beam splitter 7, on the semitransparent mirror 6. The return light incident on the semitransparent mirror 6 as such has been changed into linearly-polarized light in the P-direction by the phase shift operation of the quarter-wave plate 8. Therefore, the return light of the third laser beam is not reflected by the semitransparent mirror 6 and passes through the semitransparent mirror 6 as a control laser beam.
Reference numeral 14 denotes an astigmatism plate on which the control laser beams having passed through the semitransparent mirror 6, and which has a function of increasing the magnitude of the astigmatism caused by the semitransparent mirror 6 so as to become the magnitude suitable for generating a focus error signal, and has a function of correcting the coma aberration caused by the semitransparent mirror 6. This astigmatism plate is an optical element called an aberration correction plate.
Reference numeral 15 denotes a photodetector on which the control laser beams are applied through the astigmatism plate 14, and which includes a well-known four-divided sensor, etc., as depicted in FIG. 3, and is configured to perform a signal generating operation associated with an operation of reading signals recorded on a signal recording layer of an optical disc and a focus error signal generating operation for performing a focusing control operation by an astigmatic method through an irradiation operation of the main beam, as well as a tracking error signal generating operation for performing a tracking control operation through an irradiation operation of the two sub-beams.
As described above, when comparing the outward path of the first laser beam emitted from the laser diode 1 to the signal recording layer L1 of the first optical disc D1, the outward path of the second laser beam emitted from the two-wavelength laser diode 3 to the signal recording layer L2 of the second optical disc D2, and the outward path of the third laser beam emitted from the two-wavelength laser diode 3 to the signal recording layer L3 of the third optical disc D3, it is understood that the optical path from the polarizing beam splitter 7 to the wavelength selective element 12 is used in common thereamong.
When comparing the return path of the return light of the first laser beam reflected from the signal recording layer L1 of the first optical disc D1 to the photodetector 15, the return path of the return light of the second laser beam reflected from the signal recording layer L2 of the second optical disc D2 to the photodetector 15, and the return path of the return light of the third laser beam reflected from the signal recording layer L3 of the third optical disc D3 to the photodetector 15, it is understood that the optical path from the wavelength selective element 12 to the photodetector 15 is used in common there among.
In an optical pickup apparatus depicted in FIG. 14, all the laser beams use, in common, the outward path guiding the laser beams to the signal recording layers of the optical discs and the return path guiding the return light reflected from the signal recording layers of the optical discs to the photodetector 15, which leads to such advantages that the number of optical components can be reduced, thereby being able to not only reduce the cost of manufacturing but also miniaturize the optical pickup apparatus.
The photodetector 15 includes four-divided sensors as depicted in FIG. 3, and in FIG. 3, reference numeral 15A denotes a main-beam light-receiving portion irradiated with the return light of the main beam, and reference numerals 15B and 15C denote sub-beam light-receiving portions irradiated with the return light of the sub-beams. The photodetector with such a configuration performs: an operation of generating the focus error signal for performing the focusing control operation of condensing a laser beam onto a signal recording layer provided in an optical disc; and an operation of generating the tracking error signal for performing the tracking control operation of causing a laser beam to follow a signal track provided in the signal recording layer, however, these operations are well known and will not be described.
The operation of generating the focus error signal is performed by executing subtraction with respect to signals, obtained by adding signals acquired from diagonally arranged sensors of the four sensors configuring the main-beam light-receiving portion 15A, and a configuration is made so as to utilize a change of a laser spot shape into an oval shape occurring associated with displacement of the objective lens in a direction of a signal surface of an optical disc.
The focus control method utilizing such a focus error signal is referred to as an astigmatic method and a configuration is made such that such astigmatism is generated by the semitransparent mirror 6 and the astigmatism plate 14 depicted in FIG. 4. The rising angle of the laser beam relative to the tangential direction of the optical disc is set to 45 degrees such that the direction in which the laser spot formed by irradiation of the return light reflected from the signal recording layer of the optical disc on the main-beam light-receiving portion 15A is changed in shape becomes the diagonal direction of the four sensors.
In the optical pickup apparatus with such a configuration, if an optical configuration is common among the laser beams having three different wavelengths, such problems are caused that not only high-accuracy optical components are necessary but also an advanced assembly technique is required for assembly work etc.